1. Field of the Invention
The invention relates generally to semiconductor processing and more particularly to optimizing p-wells and n-wells for PMOS and bipolar which yield an improved BICMOS process.
2. Description of the Prior Art
BiCMOS is a semiconductor technology for high performance devices and is in widespread use. The term "BiCMOS" means there is a mixture of bipolar technology and CMOS technology in the same chip design. Using both technologies simultaneously has not been easy. Very often conflicts arise.
For example, both bipolar and PMOS use the same kind of n-well. To avoid a so-called "PMOS short channel effect," a reasonably high n-well concentration is needed around the surface area. To guarantee a good current drive of bipolar, the bottom of the n-well requires a high doping concentration. The doping concentration needs of the two technologies are at opposite ends.
Sharing of the n-wells themselves between PMOS and bipolar also presents problems. A high bottom-well concentration will increase any PMOS capacitances. A high top-well concentration will decrease the walled-emitter bipolar collector to emitter punchthrough voltage (B.sub.vces.). The latter effect will be exacerbated by n-type dopants, such as arsenic and/or phosphorus, that tend to "pile-up" in the silicon around the field oxide during field oxidation (see, FIGS. 1A and 1C, especially
region labelled as "c"). The piled-up n-type dopants lower the effective base doping, which can cause punchthrough in these regions, because the base is so easily depleted.
In prior art "advanced" BiCMOS processes there are at least three kinds of process flows being commonly used. These and two process flows of the present invention are summarized in Table I. Only the prior art process flows A, B, and C will be discussed in this section. The process flows D and E/F are detailed below in the "detailed description of the embodiments of the present invention" section.
TABLE I ______________________________________ FLOW D FLOW E/F FLOW A FLOW B FLOW C present present (prior art) (prior art) (prior art) invention invention ______________________________________ 2 masks 2 masks 2 masks 1 mask 2 masks p-well n-well n-well .... n-well mask mask mask mask p-well n-well n-well .... n-well implant implant implant implant blanket/ blanket/ .... blanket n- blanket n- self- self- well well aligned n- aligned p- implant implant well well implant implant .... .... .... .... .... field mask field mask field oxide field oxide field oxide field field p-well/ p-well/ p-well/ implant implant field mask field mask field mask field oxide field oxide p-well/ p-well/ p-well/ field field field .... .... .... blanket n .... punchthru implant .... .... .... .... .... ______________________________________
In flow "A", which requires two masks for the following described steps, an implant masking step is used for a p-well implant. An n-well implant is either blanket or self-aligned to the p-well, therefore eliminating a masking step. A masking step is used for a field implant. Each of these implantations precede field oxidation, and no high energy (.gtoreq.180 Key) implantation is required.
Flow "B", requires two masks for the following described steps, and is basically the same as flow "A", except that an n-well implant uses a masking step, while a p-well implant is blanket or self-aligned to the n-well
Flow "C", also requires two masks for the following described steps, An n-well masking step is used for an n-well implant prior to field oxidation. After field oxidation, an implantation masking step is used to adjust p-well doping for the active as well as for the field regions. The p-well implant and field implant are combined together, which requires a high energy. Generally, a retrograded p-well will be formed with this flow type.
In all the above flows, two masking steps are required to define p-wells, n-wells, and field doping. And only one kind of an n-well is available.
Therefore, there is a need for process flows using n-wells that are optimized for both bipolar and PMOS and that have no more, and preferably fewer, masking steps.